Refactors how URLs are computed.
Adds `git pull-request {repoName}/{branchNumber}`

* GitProc::PullRequest has #checkout_pull_request to checkout the
  HEAD branch of a pull request at any "remote" that has been
  configured, setting the upstream to the pull request's BASE branch
* GitHubService module no longer uses module-level instance
  variables, which were causing problems with testing
* GitHub::PullRequest can now ask for a pull request by id
* Adds ability to set the upstream/tracking on a branch
* Upgrades to Octokit 1.19 and no longer overrides the Connection
  class
* Adds module GitHubTestHelper

Closes GH-92 and GH-74.
Update README.md
Changed ChangeFileHelper to be a class instead of a module.
Major refactoring

* Made GitLib and AbstractMergeErrorBuilder classes instead of modules
* Extracted GitCofig and GitRemote out of GitLib
* Changed to use GitLogger as a logging fascade
* GitProcess now takes a GitLib instead of a directory
GitHubService refactoring.

Includes extracting GitHubService::Configuration and GitHub::CreatePullRequest.